Glutamate transporters are responsible for clearing synaptically released glutamate from extracellular space. By this action, they maintain low levels of ambient glutamate, thus preventing excitotoxic damage, and contribute to shape synaptic currents. We show that up-regulation of the glutamate transporter GLT-1 by ceftriaxone severely impaired mGluR-dependent long-term depression (LTD), induced at rat mossy fibers-CA3 synapses by repetitive stimulation of dentate gyrus granule cells. This effect involved GLT-1, since LTD was rescued by the selective GLT-1 antagonist dihydrokainate (DHK). DHK per se produced a modest but consistent decrease in fEPSP amplitude that rapidly regained control levels after washing out the drug. Moreover, the degree of fEPSP inhibition induced by the low affinity glutamate receptor antagonist g-DGG was similar during basal synaptic transmission but not during LTD, indicating that, in ceftriaxone-treated animals, LTD induction did not alter synaptic glutamate transient concentration. Postembedding immunogold studies in rats showed an increased density of gold particles coding for GLT-1a in astrocytic processes and in mossy fiber terminals; in the latter, gold particles were located near and within the active zones. In both CEF-treated KO mice and KO mice the density of gold particles in MF terminals was comparable to background levels. The enhanced expression of GLT-1 at release sites may prevent activation of presynaptic mGluRs and LTD induction. Similarly, in ceftriaxone treated animals, the magnitude of LTP induced at MF-CA3 synapses (but not at Schaffer collateral-CA1 synapses) by high frequency stimulation of granule cells in the dentate gyrus was significantly reduced. These results reveal a novel mechanism by which GLT-1 regulates synaptic plasticity in the hippocampus.